Electrochromic materials, which change color or transparency in response to an applied voltage, hold great promise for applications in smart windows, anti-glare mirrors, and low-energy displays. However, to achieve a truly transparent state, especially in metal-based systems, remains a challenge. Many existing materials suffer from limited optical contrast, slow switching speeds, or poor cycling stability.

In a study published in Angewandte Chemie International Edition, a team led by Prof. CHEN Zhongning from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a novel manganese(II)-based metallopolymer film which exhibits exceptional electrochromic performance. The film switches reversibly among three distinct states with high contrast, fast response, and remarkable cycling stability of up to 10,000 cycles.

Researchers designed a hexadentate nitrogen chelator functionalized with triphenylamine groups, which coordinates with manganese(II) ion to form an octahedral manganese(II) chelate. The half-filled d⁵ configuration of Mn(II) suppressed visible-region d-d transitions, enabling a transparent initial state. Through oxidative electropolymerization on an indium tin oxide electrode, the Mn(II) chelates were linked to form a uniform metallopolymer film.

Scanning electron microscopy of the metallopolymer film revealed a nanoporous structure conducive to fast ion transport. Spectroelectrochemistry confirmed that the film remained colorless until a voltage was applied, whereupon stepwise oxidation at Mn(II) and triphenylamine sites induced color changes: first to brown-yellow (0.9 V) and then to blue (1.3 V).

Using the metallopolymer film as the active layer, researchers constructed a solid-state electrochromic device, paired with a TiO₂ ion storage layer and a UV-cured electrolyte. The device achieved optical contrasts of 40% (brown-yellow) and 73% (blue), with rapid switching times as fast as 1.4 s for coloring and 0.4 s for bleaching. It achieved high coloration efficiencies of 337.5 cm² C^-1 and 397.1 cm² C^-1 for the two colored states, respectively.

The device demonstrated outstanding long-term stability. After 10,000 continuous switching cycles between the colorless and brown-yellow states, the optical contrast retained 80% of its initial value, surpassing most of WO₃-based and organic-polymer devices. Besides, thanks to the simple and scalable electrophysmerization process, large-area devices up to 20 × 20 cm² were constructed, which exhibited the same reversible three-state color switching.

The use of a stable Mn(II) chelate combined with electropolymerizable units provides a way for designing durable, high-performance electrochromic devices. This study demonstrates a molecular engineering strategy to overcome key limitations in electrochromic materials.